Conventionally, separation or concentration of a gas or liquid mixture containing an organic compound and an inorganic compound is performed by distillation, azeotropic distillation, solvent extraction/distillation, freeze concentration separation, adsorption, absorption, cryogenic separation and other methods according to the properties of the target substance. However, these methods are disadvantageous in that a lot of energy is required or the range of application to the target for separation or concentration is limitative.
Recently, as a separation method substituting for these methods, a membrane separation/concentration method using a membrane such as polymer membrane or zeolite membrane has been proposed. The polymer membrane, for example, a flat membrane or a hollow fiber membrane, is excellent in processability but has a drawback of low heat resistance. In addition, many of polymer membranes are low in the chemical resistance and swell when contacted particularly with an organic compound such as organic solvent or organic acid, and the range of application to the target for separation or concentration is limitative.
The zeolite membrane is usually used for separation or concentration as a zeolite membrane composite obtained by membranous formation of zeolite on a support. For example, a mixture of an organic compound and water is put into contact with the zeolite membrane composite to selectively allow permeation of water, whereby the organic compound can be separated or concentrated. Compared with distillation or separation using an adsorbent, in the separation and concentration using a zeolite membrane, the amount of energy used can be reduced and in addition, because of an inorganic material, the separation or concentration can be conducted in a wider temperature range than that with a polymer membrane. Furthermore, the membrane can be applied also to separation of a mixture containing an organic compound.
As the separation method using a zeolite membrane, in the case of liquid separation, for example, a method of concentrating an alcohol by selectively allowing permeation of water by use of an A-type zeolite membrane composite (Patent Document 1), a method of concentrating an alcohol by selectively allowing permeation of water from a mixed system of an alcohol and water by use of a mordenite-type zeolite membrane composite (Patent Document 2), and a method of separating/concentrating acetic acid by selectively allowing permeation of water from a mixed system of acetic acid and water by use of a ferrierite-type zeolite membrane composite (Patent Document 3) have been proposed.
Examples of the gas separation include separation between carbon dioxide and nitrogen discharged from thermal power plants, petrochemical industries, etc., and separation between hydrogen and hydrocarbon, between hydrogen and oxygen, between hydrogen and carbon dioxide, between nitrogen and oxygen, and between paraffin and olefin. As the zeolite membrane usable for gas separation, zeolite membranes such as A-type membrane, an FAU membrane, an MFI membrane, a SAPO-34 membrane and DDR membrane are known.
In a natural gas purification plant or a plant for biogas generation through methane fermentation of garbage, etc., separation between carbon dioxide and methane is demanded and as the zeolite membrane enabling successful separation therebetween, DDR (Patent Document 4), SAPO-34 (Non-Patent Document 1) and SSZ-13 (Non-Patent Document 2), each utilizing the molecular sieve function of zeolite, are known as a high-performance membrane.